Convergent evolution and the limits of natural selection
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Stephen Jay Gould argued that replaying the “tape of life” would result in a radically different evolutionary outcome. Some biologists and philosophers, however, have pointed to convergent evolution as evidence for robust replicability in macroevolution. These authors interpret homoplasy, or the independent origination of similar biological forms, as evidence for the power of natural selection to guide form toward certain morphological attractors, notwithstanding the diversionary tendencies of drift and the constraints of phylogenetic inertia. In this paper, I consider the implications of homoplasy for the debate over the nature of macroevolution. I argue that once the concepts of contingency and convergence are fleshed out, it becomes clear that many instances of homoplasy fail to negate Gould’s overarching thesis, and may in fact support a Gouldian view of life. My argument rests on the distinction between parallelism and convergence, which I defend against a recent challenge from developmental biology. I conclude that despite the difficulties in defining and identifying parallelism, the concept remains useful and relevant to the contingency controversy insofar as it underscores the common developmental origins of iterated evolution.